The invention relates to monocrystalline silicon diaphragms which by virtue of their electrostatic deformation capability are applicable in a wide range of uses.
In the manufacture of integrated circuits and integrated electronic devices wherein a substrate of semiconductor material such as silicon is utilized, devices which behave as inductors which are compatible with the substrate material have long been sought. No satisfactory method has previously been achieved, thus requiring the use of large scale or discrete components in conjunction with integrated circuits. The elimination of such discrete components would therefore be valuable in both the reduction of the size as well as the weight of circuits requiring inductive behaving devices. The high cost of manufacturing such hybrid circuits is a result of the manufacturing step of adding or attaching the discrete components to the integrated circuits which have already been mechanized by integrated circuit techniques; the elimination of such a step may therefore considerably reduce the cost of manufacturing such circuits.
Silicon and other semiconductor membranes of thin section have been known in the art, but it has not been heretofore discovered that the electrostatic deformation of such membranes having certain dimensions enable them to be utilized as a variable capacitance, as an electromechanical resonator (by means of the superimposition of an AC voltage upon a DC bias for creation of the electrostatic force) or for other applications wherein a small controllable or resonant movement of a diaphragm is useful.
In the prior art, for example, thin silicon diaphragms have been used as pressure sensors, but such devices have generally been manufactured in such a fashion as to form a configuration of strain gauge elements. Such an application is taught in the U.S. Pat. No. 3,697,918 to Orth et el. U.S. Pat. No. 3,814,998 to Thoma et al. shows the silicon membranes which are utilized to form a sandwich with a dielectric core; the decrease of the thickness of the inner dielectric core changes the capacitance of the sandwich. However, in none of the prior art which has contemplated the use of silicon membranes has it been recognized that the application of electrostatic attraction forces between a thin silicon membrane formed in a silicon wafer and an electrode of opposite polarity can induce both movement and electromechanical resonance.
The present invention, however, contemplates structures which are capable of exhibiting resonant behavior as well as controlled deflection in response to external forces. These structures enable the construction of improved pressure transducers, electro-optical display devices, electromechanical resonant devices such as tank circuits, and inductive devices, all of which are capable of mechanization on the same substrate as integrated circuits as conventionally manufactured and by techniques which are compatible with present integrated circuit manufacture.
Known procedures for forming thin membranes in silicon substrates require a lengthy deposition diffusion of an impurity into a portion of the substrate to a desired depth. For example, boron is known to provide etchant resistant properties when diffused into silicon above certain concentration levels. The typical procedure for forming an etchant resistant layer with this type of impurity is to diffuse boron from a gas ambient into a portion of the silicon substrate. To achieve an etchant resistant layer 1 to 3 microns thick having sufficient boron concentration to resist specific silicon etchants, it has been necessary to retain the silicon substrate in the diffusion furnace for long periods of time, in the range of 3 hours or more. A surface phase is inevitably formed on the surface of the substrate which is primarily a boro-silicate glass, i.e., a mixture of boron oxide and silicon dioxide. The boro-silicate glass is generally removed before further processing is performed on the substrate. Such procedures are incompatible with the formation of further integrated circuit components on the silicon slab, since the long deposition diffusion time required allows the boron to penetrate an oxide mask in other areas of the substrate. As a result, there will exist unwanted levels of boron impurities which can interfere with the construction of semi-conductor devices on the substrate.